1. Field of the Invention
This invention relates to a power conversion device composed of self-turn-off devices with snubber energy regenerating circuits, and more particularly to a power conversion device composed of a plurality of series connected self-turn-off devices with multistage snubber energy regenerating circuits.
2. Description of the Related Art
For power system control and/or magnetic levitation transport etc., it has now become the practice to employ such a type of circuitry as a multiple-inverter composed of a plurality of ultra-high capacity inverters, connected by a transformer, of a few tens through a few hundreds MVA class constructed. by connecting in series a plurality of self-turn-off devices such as gate turn-off thyristors (hereinbelow called GTOs). In such an ultra-high capacity GTO inverter, heat is generated with the considerable power loss resulting from resistance loss in the anode reactors and snubber circuits provided to protect GTOs. Research and development is therefore being conducted with the aim of decreasing such heat loss. As a prior art for recovering the heat loss of the snubber circuits and anode reactors to the power source end, a package snubber system for a module shown in FIG. 23 is proposed, which is disclosed in Paper No. 492 of the National convention of the Institute of Electrical Engineers of Japan published in 1993.
In FIG. 23, there are provided GTO modules 30 and 40. GTO modules 30 and 40 are composed of series connected GTOs 31 to 34 and series connected GTOs 41 to 44, respectively. There are further provided small-capacity individual snubber circuits composed of snubber diodes 31D to 34D and 41D to 44D, snubber capacitors 31C to 34C and 41C to 44C, and snubber resistors 31R to 34R and 41R and 44R, respectively. Each of the small-capacity individual snubber circuits is connected in parallel with one of GTOs 31 to 34 and 41 to 44, respectively. There are also provided package snubber circuits composed of capacitors 36C and 46C and diodes 31S and 44S, respectively. Each of the package snubber circuits is connected in parallel with one of series connected GTOs 31 to 34 in GTO module 30 and series connected GTOs 41 to 44 in GTO module 40, respectively. Anode reactors 31L and 41L are connected in series with series connected GTO modules 30 and 40 between a positive terminal P and a negative terminal N. An energy recovery circuit at an upper arm is composed of a recovery capacitor 35C which is charged through a diode 2S, a DC/DC converter 50 and a divided filter capacitor 21A. Likewise, an energy recovery circuit at a lower arm is composed of a recovery capacitor 45C which is charged through a diode 45S, a DC/DC converter 70 and a divided filter capacitor 21B.
In the arrangement of this prior art example, with ON/OFF of GTO module 30, the energy possessed by anode reactor 31L and package snubber capacitor 36C which has been heat loss is shifted into recovery capacitor 35C, and the stored energy of this recovery capacitor 35C is regenerated to divided filter capacitor 21A at the power source end using DC/DC converter 50. Likewise, with ON/OFF of GTO module 40, the energy possessed by anode reactor 41L and package snubber capacitor 45C which has been heat loss is shifted to recovery capacitor 45C, and the stored energy of this recovery capacitor 45C is regenerated to divided filter capacitor 21B at the power source end using DC/DC converter 70. Thus, the construction is such that the package snubber loss and anode reactor loss are regenerated to the power source.
However, the prior art example described above is subject to the following problem. Specifically, since an energy regeneration scheme is not devised for each individual snubber circuit, the snubber loss still exists in the form of heat loss. Accordingly, the problem of energy regeneration of a plurality of multistage snubber circuits is not at all solved. Furthermore, in addition to ON/OFF control of the main GTOs, the control of the DC/DC converters is required, resulting a new problem that the control of the power conversion device becomes complicated.